ABSTRACT Modifications of the size, shape, and number of spines is thought to be an important component of experience-dependent changes in neuronal circuits and may play an important role in the plasticity of addiction. Cellular models of activity-dependent plasticity have shown that changes in the subcellular localization of glutamate receptors is associated with a molecular reorganization of the postsynaptic density and alterations in spine morphology and/or density. The NMDA subtype of glutamate receptors play a central role in synaptic plasticity and are known targets of ethanol. Chronic ethanol consumption results in adaptive changes in neuronal function that manifest as tolerance, physical dependence and addiction. A potential adaptive mechanism we recently identified is the selective targeting of NR2B-containing NMDA receptors to the synapse. This increase is associated with, and dependent upon, a corresponding increase in the localization of the scaffolding protein PSD-95 at the postsynaptic density, and with an actin-dependent increase in the size of dendritic spines. These observations lead us to propose a molecular model for ethanol- induced plasticity at excitatory synapses in which increases in NR2B-containing NMDA receptors and PSD-95 at the postsynaptic density provides an expanded scaffolding platform for the recruitment and activation of signaling molecules that regulate spine actin dynamics, protein translation and synaptic plasticity. This renewal application will utilize biochemical, confocal imaging and electrophysiology procedures to test this hypothesis using well-defined in-vitro and in- vivo models of chronic ethanol exposure. The specific aims are to: (1) Test the hypothesis that modulation of spine actin dynamics is altered in response to chronic ethanol exposure; (2) Test the hypothesis that chronic ethanol exposure increases the size of dendritic spines; (3) Test the hypothesis that chronic ethanol exposure enhances the PSD-dependent association of translational-regulatory-proteins that modulate activity-dependent spine remodeling; (4) Test the hypothesis that the development of chronic ethanol-induced synaptic plasticity requires a PSD scaffolding-signaling complex that can support actin-based spine remodeling. This is a novel and timely proposal that is consistent with accumulating evidence that glutamatergic modulation of spine actin by the PSD plays a critical role in the plasticity of alcoholism and alcohol-related behaviors.